Process for the preparation of powder coatings of epoxide resin, polymeric polycarboxylic acid and polyol

ABSTRACT

The invention relates to a process for the preparation of powder coatings, in which a mixture of 
     A) a binder comprising an epoxide group-containing synthetic resin or a mixture of epoxide group-containing synthetic resins, and 
     B) a crosslinking agent comprising a compound which contains on statistical average at least two carboxyl groups and at least one acid anhydride group per molecule, or a mixture of such compounds, is admixed before or during the melt homogenization process with 
     C) a polyol or a mixture of polyols in such an amount that from 0.1 to 0.9 hydroxyl groups of component C) are present per acid anhydride group of component B). 
     The mixture is homogenized in the molten state and the resulting homogenized melt, after having cooled and solidified, is powdered.

The invention relates to a process for the preparation of powdercoatings, in which a mixture of

A) a binder comprising an epoxide group-containing synthetic resin or amixture of epoxide group-containing synthetic resins, and

B) a crosslinking agent comprising a compound which contains onstatistical average at least two carboxyl groups and at least one acidanhydride group per molecule, or a mixture of such compounds,

is homogenized in the molten state and the resulting homogenized melt,after having cooled and solidified, is pulverized.

The invention also relates to powder coatings prepared by this process.

The process described above is known (cf. e.g. U.S. Pat. No. 4,091,048,DE-A-2 214 650 and EP-A-299 420.

EP-A-299 420 discloses a process for the preparation of powder coatingsof the type described above, which is intended to enable the preparationof powder coatings which are stable on storage and give coating filmshaving good properties. The process described in EP-A-299 420 isdistinguished in that the compound employed as component B) has beenmodified by reaction with a polyol and contains on statistical averageat least two carboxyl groups and at least two acid anhydride groups permolecule. The powder coatings prepared by this process give coatingfilms whose properties, in particular for application in automotivefinishing, are in need of improvement.

The object of the present invention consists in the provision of aprocess for the preparation of powder coatings of the type described atthe beginning which enables the preparation of powder coatings which, incomparison with the powder coatings of the prior art, give coating filmshaving improved properties. This object is surprisingly achieved by aprocess of the type described at the beginning which is characterized inthat the mixture of A) and B) is admixed before or during the melthomogenization process with

C) a polyol or a mixture of polyols in such an amount that from 0.1 to0.9, preferably from 0.3 to 0.6, hydroxyl groups of component C) arepresent per acid anhydride group of component B).

The powder coatings prepared by the process according to the inventionare distinguished from powder coatings of the prior art in that theygive coating films having improved adhesion (especially when used asclear coats for two-coat automotive finishes) and improved resistance topremium-grade gasoline and methyl ethyl ketone.

Powder coatings are conventionally prepared by homogenizing a mixture ofcomponents A) and B) in the molten state and powdering the thusresulting homogenized melt after it has cooled and solidified. Theprocess of melting and homogenizing the mixture of components A) and B)is termed the melt homogenization process. It is usually carried out byextruding the mixture. Depending on the desired powder coatingformulation, the mixture of components A) and B) can be admixed beforeor during the melt homogenization process with further conventionalpowder coating constituents, such as, for example, one or morecrosslinking catalysts (for example quaternary ammonium salts,quaternary phosphonium salts, phosphines, imidazolines and metal salts,cf. also EP-A-299 420, page 4, line 51 and line 52), one or morepigments, one or more light stabilizers, one or more degassing agents(for example benzoin), one or more leveling assistants (cf. e.g.EP-A-299 420, page 5, line 7 to 14), one or more additional binders, oneor more additional crosslinking agents etc. The melt obtained at the endof the melt homogenization process is powdered after it has cooled andsolidified.

The process described above for the preparation of powder coatings iswell known and requires no further discussion (cf. e.g. also H. Kittel,Lehrbuch der Lacke und Beschichtungen [Textbook of Paints and Coatings],Volume VIII, Part 2, Verlag W. A. Colomb in HeenemannVerlagsgesellschaft mbH, Berlin and Oberschwandorf, 1980).

It is essential to the invention that the mixture of components A) andB) is admixed before or during the melt homogenization process with C) apolyol or a mixture of polyols in an amount such that from 0.1 to 0.9,preferably from 0.3 to 0.6, hydroxyl groups of component C) are presentper acid anhydride group of component B). Component C) is preferablyadded to the mixture of components A) and B) before the beginning of themelt homogenization process. If component C) is added during the melthomogenization process, it must then be ensured that the addition ismade at a suitable point in time such that there is still sufficienttime for homogeneous incorporation of component C).

The duration of the melt homogenization process and the temperatures tobe employed depend on a number of parameters (for example on the natureof the machine employed, on the glass transition temperature of thebinder, etc.), and the person skilled in the art--as is also the case inthe processes of the prior art--must optimize these parameters with theaid of routine experiments. The mixture to be homogenized should ingeneral not be heated above 130° C., preferably not above 120° C. andparticularly preferably not above 110° C., during the melthomogenization process.

In the process according to the invention component A) can, inprinciple, be any epoxide group-containing synthetic resin which can beemployed for the preparation of powder coatings, or a mixture of suchsynthetic resins. Examples of epoxide group-containing synthetic resinswhich can be employed are epoxide group-containing polyacrylate resins,polyglycidyl ethers of aliphatic or cycloaliphatic alcohols, such as,for example, ethylene glycol, diethylene glycol, 1,2-propylene glycol,1,4-butylglycol, 1,2-cyclohexanediol, 1,4 cyclohexanediol,1,2-bis(hydroxy-methyl)cyclohexane and hydrogenated bisphenol A,polyglycidyl ethers of polyphenols such as, for example, bisphenol A,1,1-bis(4-hydroxyphenyl)ethane and2-methyl-1,1-bis(4-hydroxyphenyl)propane and the epoxidegroup-containing compounds which are listed in U.S. Pat. No. 4,102,942in column 3, line 1 to 16.

Component A) employed is preferably an epoxide group-containingpolyacrylate resin or a mixture of epoxide group-containing polyacrylateresins.

An epoxide group-containing polyacrylate resin is understood to mean apolymer which can be prepared by copolymerization of at least oneethylenically unsaturated monomer containing at least one epoxide groupin the molecule with at least one further ethylenically unsaturatedmonomer containing no epoxide group in the molecule, at least one of themonomers being an ester of acrylic acid or methacrylic acid. Epoxidegroup-containing polyacrylate resins are known (cf. e.g. EP-A-299 420,DE-B-22 14 650, U.S. Pat. No. 4,091,048 and U.S. Pat. No. 3,781,379).

Examples of ethylenically unsaturated monomers containing at least oneepoxide group in the molecule are glycidyl acrylate, glycidylmethacrylate and allyl glycidyl ether.

Examples of ethylenically unsaturated monomers containing no epoxidegroup in the molecule are alkyl esters of acrylic and methacrylic acidwhich contain 1 to 20 carbon atoms in the alkyl radical, especiallymethyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylateand 2-ethylhexyl methacrylate. Further examples of ethylenicallyunsaturated monomers which contain no epoxide groups in the molecule areacids such as, for example, acrylic acid and methacrylic acid, aromaticvinyl compounds such as styrene and vinyltoluene, nitriles such asacrylonitrile and methacrylonitrile, vinyl and vinylidene halides, suchas vinyl chloride and vinylidene fluoride, vinyl esters, for examplevinyl acetate, and hydroxyl group-containing monomers, such as forexample hydroxyethyl acrylate and hydroxyethyl methacrylate. When thepolyacrylate resins are prepared using monomers which carry functionalgroups which are potentially capable of reacting with the epoxy groups,it must be ensured that the amounts in which the monomers are employed,or the reaction conditions chosen, are such that self-crosslinking ofthe polyacrylate resin either does not occur at all or only occurs to aslight extent.

The epoxide group-containing polyacrylate resin usually has an epoxideequivalent weight of from 350 to 2000, preferably from 450 to 1500 andparticularly preferably from 500 to 1350, a number-average molecularweight (determined by gel permeation chromatisism [sic] using apolystyrene standard) of from 500 to 100,000, preferably from 500 to20,000, particularly preferably from 1000 to 10,000 and veryparticularly preferably from 1000 to 5000, and a glass transitiontemperature (T_(G)) of from 30° to 80° C., preferably from 35° to 70° C.and particularly preferably from 40° to 60° C.

The epoxide group-containing polyacrylate resin can be preparedaccording to generally well-known methods, by free radicalpolymerization.

In the process according to the invention component B) can, inprinciple, be any compound which contains on statistical average atleast two carboxyl groups and at least one acid anhydride group permolecule, or a mixture of such compounds. Component B) is in particularpolymeric anhydrides of polycarboxylic acids, preferably polymericanhydrides of dicarboxylic acids and particularly preferably polymericanhydrides of cycloaliphatic or aliphatic dicarboxylic acids, thedicarboxylic acids containing from 3 to 20, preferably from 6 to 12carbon atoms per molecule, or mixtures of such polymeric anhydrides.Specific examples of particularly preferred polymeric anhydrides arepoly(adipic anhydride), poly(azeleic anhydride), poly(sebacic anhydride)and poly(dodecanedioic anhydride), and polymeric anhydrides derived frommixtures of adipic acid and/or azeleic acid and/or sebacic acid and/ordodecanedioic acid. Component B) is very particularly preferablypoly(dodecanedioic anhydride).

Components A) and B) are mixed together in such a quantitative ratiothat from 1.5 to 0.5, preferably from 1.3 to 0.7 and particularlypreferably from 1.2 to 0.8 equivalents of carboxyl groups of componentB) are present per equivalent of epoxide groups of component A).

In the process according to the invention component C) can, inprinciple, be any polyol or a mixture of polyols. A polyol is understoodto mean an organic compound which contains on statistical average morethan one hydroxyl group per molecule. The polyols preferably employed ascomponent C) contain on statistical average at least two hydroxyl groupsper molecule. Examples of polyols which can be used as component C) arelow molecular weight polyols, such as for example trimethylolpropane,pentaerythritol, hexanediol, 2,2-dimethylolpropionic acid, glycerol,sorbitol etc., and higher molecular weight polyols such as, for example,polyester-polyols, polyether-polyols, polyurethane-polyols, celluloseacetobutyrate, polycaprolactone-polyol etc. Component C) is preferablytrimethylolpropane, hexanediol, 2,2-dimethylolpropionic acid,pentaerythritol, or a mixture of these polyols. Component C) is veryparticularly preferably trimethylolpropane.

The powder coatings prepared by the process according to the inventioncan be applied to any desired substrates, for example metal, glass orplastic, using application methods suitable for powder coatings,preferably an electrostatic method. The powder coatings prepared by theprocess according to the invention are particularly suitable as powderclear coats for the preparation of the clear coat in two-coat finisheswhich can be obtained by initially applying a pigmented base coat,preferably one pigmented with metal flakes, for example aluminum flakes,and applying a powder clear coat over the base coat thus obtained.Suitable base coats are not only water-thinnable base coats but alsobase coats containing exclusively organic solvents and/or diluents. Suchtwo-coat finishes are employed in particular in automotive finishing,for example in the preparation of metallic effect finishes.

The powder coatings prepared by the process according to the inventionare usually baked at from 130° to 180° C.

The invention is described in more detail in the examples which follow.All parts and percentages are by weight unless expressly statedotherwise.

1. Preparation of component A)

A mixture of 37.06 parts by weight of methyl methacrylate, 14.40 partsby weight of glycidyl methacrylate, 9.00 parts by weight of n-butylacrylate and 6.54 parts by weight of styrene are added at 120° C. over aperiod of 4 hours to 30.0 parts by weight of xylene. Commencing with theaddition of the monomer mixture 3.0 parts of tert-butylper-2-ethylhexanoate (TBPEH; manufacturer: Peroxid-Chemie) are addedover a period of 4.5 hours. During the addition of the monomer mixtureand of the peroxide the reaction temperature is 120° C. This temperatureis maintained for a further hour after having completed the addition ofthe peroxide. The xylene is then removed under reduced pressure, and thesynthetic resin is heated to 180° C. and discharged from the reactionvessel.

2. Preparation of component B)

67.2 parts by weight of dodecanedioic acid together with 29.8 parts byweight of acetic anhydride are weighed out and charged to a reactionvessel fitted with a reflux condenser. The mixture is heated slowlyuntil reflux begins and left for 3 hours under reflux. The acetic acidwhich has formed is then distilled off. A further 3.00 parts by weightof acetic anhydride are then added to the reaction product, and themixture is heated for one hour at reflux temperature. Finally the aceticacid which has formed is distilled off and the reaction product isdischarged from the reaction vessel at 90°.

3. Preparation according to the invention of a powder coating

1095 parts by weight of component A) prepared as in section 1. arecomminuted and premixed in a precutting mill together with 303 parts byweight of component B) prepared as in section 2., 23.8 parts by weightof trimethylolpropane (component C)), 5.9 parts by weight of benzoin,5.9 parts by weight of a leveling assistant (Perenol® F40, manufacturer:Henkel KGaA) 35.4 parts by weight of a first light stabilizer (Tinuvin®900, manufacturer: Ciba Geigy) and 23.6 parts by weight of a secondlight stabilizer (Tinuvin® 144, manufacturer: Ciba Geigy). The resultingmixture is then extruded in a Buss Ko-kneader (type PLK 46) at 60revolutions/min, the temperature in the center of the extrusion chamberbeing 90°-110° C. The extrudate is cooled rapidly to room temperature,milled in an impact mill (ACM 2L from Hosokawa MikroPul) to a powderhaving an average particle diameter of from 30-40 μm, and screenedthrough a sieve having a pore size of 125 μm.

4. Production of a two-coat finish

A commercial, water-thinnable base coat containing polyurethane,polyester and melamine resin and pigmented with aluminum flakes isapplied to a phosphatized steel panel coated with a commercialelectrodeposition coating and a commercial filler so that a dry filmthickness of from 12 to 15 μm is obtained. The applied base coat isdried for 10 minutes at room temperature and 10 minutes at 80° C. Thesteel panel coated in this way is then coated electrostatically with thepowder coating prepared as in section 3. so that a film thickness offrom 50 to 60 μm is obtained. The base coat and powder clear coat arefinally baked for 20 minutes at 160° C.

5. Comparative Example

The procedure as described under section 1. to 4. is followed with thesole exception that the trimethylolpropane (component C)) is reacted ina preliminary reaction with component B). 874 parts by weight ofcomponent B) prepared as in section 2. together with 97.8 parts byweight of trimethylolpropane are added and the reaction mixture is heldfor 1.5 hours at 130° C. 303 parts by weight of this reaction productare then comminuted and premixed in a precutting mill, as describedunder section 3., together with 1095 parts by weight of component A)prepared as in section 1., 5.9 parts by weight of benzoin, 5.9 parts byweight of a leveling assistant (Perenol® F40, manufacturer: Henkel KGaA)35.4 parts by weight of a first light stabilizer (Tinuvin® 900,manufacturer: Ciba Geigy AG) and 23.6 parts by weight of a second lightstabilizer (Tinuvin® 144; manufacturer: Ciba Geigy AG). The resultingmixture is extruded as described under section 3. and milled to apowder. Coating 10 films prepared in analogy to section 4. exhibit apoorer adhesion and a significantly poorer resistance to premium-gradegasoline and methyl ethyl ketone than the coating films prepared usingthe powder coating prepared in accordance with the invention.

We claim:
 1. Process for the preparation of powder coatings, comprisingthe steps of homogenizing, in the molten state, a mixture of:A) a binderselected from the group consisting of epoxide group-containing syntheticresins and mixtures thereof, and B) a crosslinking agent selected fromthe group consisting of compounds which contain on statistical averageat least two carboxyl groups and at least one acid anhydride group permolecule, and mixtures of such compounds, and cooling and solidifyingthe resulting homogenized melt to form a solidified coating, thenreducing the solidified coating to a powder, wherein the mixture of A)and B) is admixed before or during the melt homogenization process with:C) a compound selected from the group of polyols consisting ofhexanediol, 2,2-dimethylolproprionic acid, trimethylolpropane,pentaerythritol and mixtures thereof in such an amount that from 0.1 to0.9 hydroxyl groups of component C) are present per acid anhydride groupof component B).
 2. Powder coatings prepared according to the process ofclaim 1.